Somatotropin or growth hormone (“GH”) is a mammalian protein comprising a class of tropic hormones synthesized and secreted in the brain by the major gland of the endocrine system, the adenohypophysis. The secretion of GH and other tropic hormones by the adenohypophysis regulates the activity of cells in other endocrine glands and tissues throughout the body. Specifically, GH is secreted by somatotrophs of the anterior pituitary gland and functions to stimulate the liver and other tissues to synthesize and secrete IGF-1, a protein that controls cell division, regulates metabolic processes and exists in a free state or binds to one of six other proteins designated as IGFBP-1 through 6. The secretion process itself is modulated by opposing actions of somatoliberin (promoting GH release) and somatostatin (inhibiting GH release).
Human growth hormone (“hGH”) serves as a critical hormone in the regulation of cell and organ growth and in physiological function upon various stages of aging. For example, overproduction of hGH results in gigantism in children and acromegaly in adults, whereas under-production leads to dwarfism in children [Mauras et al., J. Clin. Endocrinology and Metabolism, 85(10), 3653-3660 (2000); Frindik et al., Hormone Research, 51(1), 15-19 (1999); Leger et al., J. Clin. Endocrinology and Metabolism, 83(10), 3512-3516 (1998)], Turner's Syndrome (females only) [Bramswig, Endocrine, 15(1), 5-13 (2001); Pasquino et al., Hormone Research, 46(6), 269-272 (1996)] and chronic renal insufficiency [Carroll et al., Trends in Endocrinology and Metabolism, 11(6), 231-238 (2000); Ueland et al., J. Clin. Endocrinology and Metabolism, 87(6), 2760-2763 (2002); Simpson et al., Growth Hormone & IGF Research, 12, 1-33 (2002)]. In adults, hGH deficiency can affect metabolic processing of proteins, carbohydrates, lipids, minerals and connective tissue and can result in muscle, bone or skin atrophy [Mehls and Haas, Growth Hormone & IGF Research, Supplement B, S31-S37 (2000); Fine et al., J. Pediatrics, 136(3), 376-382 (2000); Motoyama et al., Clin. Exp. Nephrology, 2(2), 162-165 (1998)]. Other hGH deficiency disorders characterized by growth failure include AIDS wasting syndrome [Hirschfeld, Hormone Research, 46, 215-221 (1996); Tritos et al., Am. J. Medicine, 105(1), 44-57 (1998); Mulligan et al., J. Parenteral and Enteral Nutrition, 23(6), S202-S209 (1999); Torres and Cadman, BioDrugs, 14(2), 83-91 (2000)] and Prader-Willi syndrome [Ritzen, Hormone Research, 56(5-6), 208 (2002); Eiholzer et al., Eur. J. Pediatrics, 157(5), 368-377 (1998)].
To date, treatment regimens for hGH deficiency in humans focus primarily on subcutaneous injection of purified hGH made by recombinant DNA technology. The therapeutic is packaged as either a solution in a cartridge or a lyophilized powder requiring reconstitution at the time of use. The frequency of injection varies depending on the disease being treated and the commercially available product being used.
The use of subcutaneous administration as a rapid delivery route for hGH is necessitated by the inherent instability of the protein in solution. That instability results from cleavage of critical intramolecular crosslinks at specific positions within the amino acid sequence of the protein, which in turn disrupts the essential three-dimensional structure recognized by and associated with cellular surfaces in the patient. The mechanism for hGH cleavage or degradation is orchestrated primarily by oxidation of methionine residues or deamidation of aspartic acid residues upon dissolution, thereby rendering the protein inactive.